1. Technical Field of the Invention
The present invention relates generally to a probe for detecting the Barkhausen signals from a ferromagnetic sample when it is exposed to a varying external magnetic field.
2. Background and Objects of the Invention
The Barkhausen noise method is based on the concept of ferromagnetic domains. Each domain is magnetized along a certain crystallographic easy direction of magnetization. Domains are separated from one another by walls within which the direction of the magnetization vectors usually turns 180 or 90 degrees. When a magnetic field or mechanical stress is applied to a ferromagnetic material, changes take place in its domain structure by abrupt movement of domain walls or rotation of domain magnetization vectors. The abrupt movements of the domain walls result in step changes in the magnetization curve of a ferromagnetic material that can be observed by sensitive measurements. If a coil of conducting wire is placed near a ferromagnetic material while its domain walls are moving, the resulting change in the magnetization of the ferromagnetic material will induce electrical pulses in the coil. These electrical pulses, which are stochastic in nature, make up what is called Barkhausen noise. The frequency content of the Barkhausen noise gives rise to a power spectrum that will start at the magnetizing frequency and extend up to several megahertz. The magnetic signals are exponentially damped as a function of the distance they have to travel from within the material to its surface. The magnetic pulses undergo damping due to counter magnetic fields set up within the material by transient eddy currents. The amount of damping determines the depth from which information can be obtained. The operating frequency of a Barkhausen noise probe is a function of its type, size and geometry. It is an object of this invention to provide a Barkhausen noise probe that has high sensitivity at very low frequencies.
Sakamoto et al., in U.S. Pat. No. 6,073,493 described a Barkhausen apparatus with a U-shaped magnetic excitation head and a magnetic detection head consisting of a wire wound air-core coil. The U-shaped excitation head is made of soft magnetic material and an excitation coil. The apparatus of Sakamoto is designed to diagnose the fatigue life structural steel work using the root-mean-square (RMS) voltage or voltage amplitude value of the Barkhausen noise.
Kohn et al., U.S. Pat. No. 5,619,135 described a steel hardness measuring system that comprises an energizing yoke, a Hall probe, a Barkhausen signal sensor and a signal processing unit. The coil-wound energizing yoke is used to provide the magnetic flux to generate the Barkhausen noise within the sample. The Hall probe was provided to measure the tangential magnetic field intensity H at the surface of the sample to provide for calibrations of the device of U.S. Pat. No. 5,619,135. The Barkhausen signal sensor in this case was of one of a variety of known coil type sensors designed to sense a changing magnetic flux. The invention of this utility patent application has only to do with the probe for creating and detecting the Barkhausen noise.
Perry, in U.S. Pat. No. 5,166,613 invented a system for identifying and measuring stress at specific locations within a ferromagnetic material by the production and detection of Barkhausen noise during magnetization of the material. The apparatus of Perry consists of a signal generator for generating a combined cyclic and spatially varying magnetic field signal using two electromagnets located on opposing faces of a sample. He teaches the use of acoustical Barkhausen noise sensors located near the two electromagnets coupled to computing and analysis capability for analyzing the acoustic Barkhausen noise. The acoustic sensors, which must be attached to the sample, are designed to operate in the frequency range from 75 to 450 kHz. These sensors provided for greater penetration into the thickness of a test sample than what is normally associated with wound coils. The aim of this invention is to provide a probe that will provide for the sensing of Barkhausen noise from depths not possible with the typical wire wound coils. A still further aim is to provide a sensor that will not have to be physically attached to the surface of a test sample.
Typically the sensor used to detect Barkhausen noise may comprise any of a variety of known coil sensor types. Examples of prior art Barkhausen signal sensors include encircling coils, surface pancake coils, ferrite core surface coils, etc. All detection coils are based upon Faraday""s Law, which states that the total emf induced in a closed circuit, is equal to the time rate of decrease of the total magnetic flux linking the circuit. That is, the induced emf is a function of the area of the coil and the rate of change of the magnetic flux at the coil. This suggests that a wire wound coil has reduced sensitivity at low frequencies. It is an object of this invention to provide a Barkhausen noise generator and detector with increased sensitivity at lower frequencies. It is a further object to provide a Barkhausen generating and sensing probe that can sample a ferromagnetic material at depths not available with coil-based Barkhausen noise detectors. It is an additional object of this invention to provide a Barkhausen probe that can be used with commonly available signal conditioning and analysis systems to measure residual stress, detect faults in structures and other material properties. Other objects of the probe of this invention will become obvious during the course of the description of the probe.
Pasley, R., Barkhausen Effectxe2x80x94An Indication of Stress, Material Evaluation, Vol. 28, No 7, July 1970, pp. 157-161.
Dhar, A., Jagadish, C., and Atherton, D. L., Using the Barkhausen Effect to Determine the Easy Axis of Magnetization in Steels, Material Evaluation, October 1992, pp. 1139-1141.
Francino, P., and Tiitto, K., Evaluation of Surface and Subsurface Stresses with Barkhausen Noise: A Numerical Approach, Practical Applications of Residual Stress Technology, Proceedings of the Third International Conference, Indianapolis, Ind. May 15-17, 1991.
A Barkhausen probe comprising: a magnetizing fields generator; a magnetoresistive magnetic field sensor; a direct current power supply for biasing the magnetoresistive magnetic field sensor; and signal conditioning electronics is described. The Barkhausen probe is capable of generating and sensing the Barkhausen noise emanating from the surface of a cyclic magnetized specimen. The conditioned Barkhausen noise output of the signal conditioning electronics is usable as input to various analysis systems.